1. Field of the Invention
The present invention relates to a shielded composite magnetic lens and deflection yoke arrangement for an electron beam column. More particularly, the invention concerns such a structure particularly adapted for use with an electron beam column capable of high resolution scanning, i.e. in the order of 20,000 lines in a field 0.2 inches square, wherein the magnetic lens has relatively long focal length, i.e. of at least one inch.
2. Description of the Prior Art
Electron beam columns are well known in the prior art as they comprise portions of electron microscopes and probe forming systems adapted for structure and materials analysis. Furthermore, electron beam columns have utility in systems for microfabrication of electrical components such as large scale integrated semiconductor circuits. For example, U.S. Pat. No. 3,644,700, issued Feb. 22, 1972, to Kruppa et al describes an electron beam column adapted to form desired patterns on semiconductor wafers.
Electron beam columns utilized in connection with microfabrication applications generally include an electron beam source, a condenser lens, alignment stages, demagnification lens stages, a projection lens, a deflection unit, and a target stage, arranged in well known fashion.
A projection lens as utilized in an electron beam column is usually a magnetic lens, including magnetic pole pieces and image generating coils through which an electron beam is directed or focused upon the target. Thus, the magnetic field produced within the lens acts to focus the electrons within the beam in a manner analogous to the focusing achieved by a physical lens within a light optical system. An example of a magnetic lens utilized for this purpose is described in U.S. Pat. No. 3,659,097, issued Apr. 25, 1972, to Richard Bassett et al.
In the electron beam columns described in the above mentioned patents, it is conventional to locate a deflection yoke proximate the projection lens in order to deflect the electron beam back and forth across the target in a scanning mode as required for particular applications. Certain electron beam columns in the prior art are constructed with the deflection yoke or yokes physically arranged to deflect the beam before or after the final projection lens depending upon desired applications. Moreover, recently designed systems have included a deflection yoke located within the pole pieces of the magnetic lens, again for particular purposes.
Depending upon the relative location of the projection lens with respect to the deflection yoke, the lens is found to exhibit certain aberration characteristics. For example, performing the deflection operation before the electron beam passes through the projection lens enables construction of a system having a projection lens with a relatively short focal length. This tends to reduce on axis aberration in the lens and provide a high resolution capability to the system. A disadvantage is that the deflection angle and all deflection aberrations in the lens increase rapidly with increasing field coverage, thereby limiting the practical operation of the beam column to relatively small fields of view.
On the other hand, performing deflection after the electron beam passes through the projection lens eliminates off axis aberrations of the lens and reduces the total deflection aberrations, thereby permitting large scale coverage. However, the longer focal length required of the projection lens for such placement of the deflection yoke results in relatively poor resolution within the system and in substantial on axis aberrations of the lens.
The disadvantages of deflection yoke placement before or after the projection lens are minimized if the deflection yoke is incorporated within the beam column at a location near the center between the pole pieces of the projection lens. In this configuration, the lens field and the deflection field are superimposed and some of the deflection aberrations of the yoke are compensated by the corresponding off-axis aberrations of the lens.
However, in all three cases of yoke placement the deflection yoke is located proximate to the pole pieces of the lens and the dynamically changing deflection field will interact with conductive pole pieces and cause eddy currents which result in pattern distortions.
Furthermore, to incorporate the deflection yoke between the pole pieces of the lens, the lens normally has to be constructed with a large pole piece gap, which typically results in high astigmatism caused by winding asymmetry of the lens coil. Accordingly, a need exists in the art for electron beam columns with projection lenses constructed with wide pole piece gaps and long focal length wherein static astigmatism is minimized. A need also exists for a beam column wherein eddy current pattern distortions are reduced.